A femtocell BS refers to a BS type which may be installed in a shadow area not covered by a macrocell BS. The femtocell BS is a miniature version of the macrocell BS and may perform most of the functions of the macrocell BS. The femtocell BS includes an independently operating network construction and much more femtocell BSs than relay BSs will be installed downtowns or in indoor spaces. Therefore, a list of femtocell BSs is not included in a list of neighbor BSs transmitted by a BS to an MS because the femtocell BS list occupies much information.
FIG. 1 illustrates the configuration of a wireless communication system to which a femtocell BS is added.
As illustrated in FIG. 1, a wireless communication system to which a femtocell BS is added includes a femtocell BS 110, a macrocell BS 120, a Femto Network Gateway (FNG) 130, an Access Service Network (ASN) 140, and a Connectivity Service Network (CSN) 150. The macrocell BS 120 refers to a general BS of a conventional wireless communication system.
The femtocell BS 110 is a miniature version of the macrocell BS 120 and may perform most functions of the macrocell BS 120. The femtocell BS 110 is directly connected to a Transmission Control Protocol/Internet Protocol (TCP/IP) network and independently operates like the macrocell BS 120. The femtocell BS 110 has coverage of about 0.1 to 30 m and one femtocell BS may accommodate 10 to 20 MSs. The femtocell BS 110 may use the same frequency as the macrocell BS 120 or use different frequencies from the macrocell BS 120.
The femtocell BS 110 is connected to the macrocell BS 120 through an R1 interface to receive a downlink channel of the macrocell BS 120 and to transmit a control signal to the macrocell BS 120.
The femtocell BS 110 may cover an indoor space or a shadow area not covered by the macrocell BS 120 and may support a high data transmission rate. The femtocell BS 110 may be installed in an overlay form within a macro cell or in a non-overlay form in an area uncovered by the macrocell BS 120.
The femtocell BS 110 is classified into two types: a Closed Subscriber Group (CSG) femtocell BS and an Open Subscriber Group (OSG) femtocell BS. The CSG femtocell BS groups MSs which can access the CSG femtocell BS and assigns a CSG IDentification (ID) to the groped MSs. Only the MSs to which the CSG ID is assigned can access the femtocell BS. The OSG femtocell BS refers to a BS that all MSs can access.
The FNG 130 is a gateway for controlling the femtocell BS 110 and is connected to the ASN 140 and the CSN 150 through an Rx interface and an Ry interface, respectively. The femtocell BS 110 may be served by the CSN 150 through the FNG 130. A service for authentication and IP multimedia subsystem (IMS) functions may be provided by the CSN 150 to MSs connected to the femtocell BS 110. The CSN 150 provides connections of application services such as Internet or voice over Internet protocol (VoIP) to MSs. The ASN 140 controls the macrocell BS 120 and manages connections of the macrocell BS 120 and the CSN 150.
As described above, in the wireless communication system in which the femtocell BS is added, a distance between an MS and a BS is short and hence a Signal-to-Interference plus Noise ratio (SINR) is increased, thereby improving communication performance. However, since BSs are densely distributed, significant interference between the BSs may occur.
To reduce the interference between BSs, BSs of a conventional wireless communication system use frequency resources, electric power resources, code resources, etc. In an interference control method using the frequency resources, different frequencies are allocated to neighbor BSs so that the BSs are not physically affected by the interference. In an interference control method using the electric power resources, BSs are not allowed to use an electric power above a predetermined level, thereby preventing severe interference between BSs or between MSs. In an interference control method using the code resources, different codes or quasi-orthogonal codes are allocated to BSs and the respective BSs transmit signals by multiplying the allocated unique codes, thereby reducing interference.
Among the aforementioned three methods, the interference control method using frequency resources may be divided into a method for completely discriminately allocating frequencies to BSs, and a method for partially overlapping allocation of frequencies to BSs, in which a user in a cell edge can use a discriminated frequency. An interference control method in a combination of the methods using the frequency resources, power resources, and code resources instead of using an individual method is also widely used.
A recent wireless communication system has developed toward a tendency to ensure high transmission rate and service quality for multimedia support in addition to a voice call. To ensure such high transmission rate and service quality, it is important to introduce recent physical layer and Medium Access Control (MAC) techniques and to improve an SINR which is a basis of wireless communication. The most basic method for improving SINR is to shorten a distance between an MS and a BS. Namely, a large number of BSs should be densely distributed in order to improve SINR. Recently, many users install a Wireless Local Area Network (WLAN) BS in their households. This indicates a tendency to improve communication performance by increasing SINR. A Femto Access Point (FAP) called a miniature type BS is a BS, the size of which is reduced to the level of WLAN and the cost of which is lowered. With the emergence of such a FAP, future wireless communication systems are expected to have more densely distributed BSs.
A conventional interference control method has performed a function for allocating unique frequencies or code resources to BSs so that a central controller can reduce interference between neighbor BSs. When considering current circumstances in which wireless communication systems are rapidly developed, a future wireless communication system will have lots of BSs which are densely distributed. Although such a central method has an advantage of effectively eliminating interference between BSs, it also has a disadvantage in terms of scalability.
In circumstances in which computing power is insufficient as in the FAP and a plurality of FAPs or radio LAN BSs are distributed without any planning by a communication enterpriser, the conventional central method demands a large amount of calculation performed by BSs and therefore effective interference control is difficult.
Furthermore, unlike a WLAN which is intended to be deployed without any planning, the FAP will operate in a cellular network system based on cooperation, thereby causing more interference problems. Since the capacity of a future femtocell BS may be increased to accommodate 10 or more MSs, a new scheduling method, a new frame structure, etc. are required, which can reduce interference between BSs and can improve communication performance.